Add "visitor" plugins support (#1454)

Visitor is a simple pattern which helps to avoid many type checks
and provide both "perItem" and "perItemReverse" functionality without
fragmentation.

The most important case is an ability to define state which in many
plugins specified either on module level or by polluting `params`.

In this diff I added visit and detachFromParent utilities and refactored
new mergeStyles plugin with it.

Also fixed bug when cdata content is merged into "text" node which is
not always valid.

lib/xast.js

@@ -51,3 +51,36 @@ const traverse = (node, fn) => {
   }
 };
 exports.traverse = traverse;
+
+const visit = (node, visitor) => {
+  const callbacks = visitor[node.type];
+  if (callbacks && callbacks.enter) {
+    callbacks.enter(node);
+  }
+  // visit root children
+  if (node.type === 'root') {
+    // copy children array to not loose cursor when children is spliced
+    for (const child of node.children) {
+      visit(child, visitor);
+    }
+  }
+  // visit element children if still attached to parent
+  if (node.type === 'element') {
+    if (node.parentNode.children.includes(node)) {
+      for (const child of node.children) {
+        visit(child, visitor);
+      }
+    }
+  }
+  if (callbacks && callbacks.exit) {
+    callbacks.exit(node);
+  }
+};
+exports.visit = visit;
+
+const detachNodeFromParent = (node) => {
+  const parentNode = node.parentNode;
+  // avoid splice to not break for loops
+  parentNode.children = parentNode.children.filter((child) => child !== node);
+};
+exports.detachNodeFromParent = detachNodeFromParent;